The Generator Effect (Oxford AQA IGCSE Physics)
Revision Note
Written by: Dan Mitchell-Garnett
Reviewed by: Caroline Carroll
The Generator Effect
Inducing a potential difference in a circuit
The process of generating a potential difference in a conductor using a magnetic is called electromagnetic induction, or the generator effect
There are two methods of inducing a potential difference in a conductor:
Moving a conductor within a fixed magnetic field
Placing a conductor in a changing magnetic field
If the conductor is part of a complete circuit then the induced potential difference produces a current in that circuit
Moving the electrical conductor
When a conductor (such as a wire) is moved perpendicular to the direction of the field lines of a magnetic field (which are fixed) the wire cuts through the field lines
A potential difference is induced in the circuit which creates the current
An ammeter detects the current in the circuit
Moving an electrical conductor in a magnetic field
Moving the magnetic field
As the magnet moves through a fixed coil, the field lines cut through the turns on the coil
This generates a potential difference in the coil and induces a current
Moving the magnetic field relative to the conductor
AC generator & DC dynamo
The generator effect can be used to:
Generate ac in an alternator
Generate dc in a dynamo
A simple alternator is a type of generator that uses mechanical work to produce an alternating current
Diagram of an alternator
A rectangular coil is forced (e.g. by rising steam or wind rotating a turbine) to spin in a uniform magnetic field
The coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings
The slip rings and brushes provide a continuous connection between the coil and the ammeter
When the coil rotates continuously in the same direction:
The coil cuts through the magnetic field lines, so a potential difference (and therefore current) is induced in the coil
The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins
This is because the induced potential difference in the coil repeatedly changes its direction
This continues as long as the coil keeps turning in the same direction
The induced potential difference and the current alternate because they repeatedly change direction / polarity
Alternating current produced by alternator
A dynamo is a direct current (d.c.) generator
A simple dynamo is the same as an alternator except that the dynamo has a split ring commutator instead of two separate slip rings
Diagram of a dynamo
As the coil rotates, it cuts through the field lines
This induces a potential difference between the end of the coil
The split ring commutator changes the connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction
This happens each time the coil is perpendicular to the magnetic field lines
Therefore, the induced potential difference does not reverse its direction as it does in the alternator
Instead, it varies from zero to a maximum value twice each cycle of rotation, and never changes polarity (positive to negative)
This means the current is always positive (or always negative)
Direct current produced by dynamo
Worked Example
A coil of wire is connected to a sensitive voltmeter. When a magnet is pushed into the coil the needle on the voltmeter will deflect to the right as shown in the diagram below.
What will happen to the pointer on the voltmeter when the magnet is stationary in the centre of the coil?
A The needle will deflect to the left
B The needle will deflect to the right
C There will be no deflection of the needle
D The needle will deflect to the left and then to the right
Answer: C
C is correct because there the magnet is stationary
This means there is no relative movement between the coil and the magnetic field, therefore no magnetic field lines are being cut
If the magnetic field lines are not being cut then there will not be a potential difference induced
A, B & D are incorrect because a deflection on the voltmeter would indicate that a potential difference has been induced
This could only happen if there was relative movement between the coil and the magnetic field
Examiner Tips and Tricks
While it is valuable to be aware of the inner workings of the alternator and dynamo, you will not be asked to recall details about slip rings and split ring commutators. Just make sure you have an understanding of how the system works overall.
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